Signal lamps, such as railroad wayside signal lights, are critical pieces of safety equipment. Multiple fatal accidents have occurred over the years as a result of signal lamp failure. The history of signal lights in the railroad industry is particularly illustrative of the challenges associated with developing and implementing technological changes to improve safety and reliability.
Looking at the United States, for example, the national railroad network was already quite extensive by the time widespread electrification became practical. Replacing mechanical signals with incandescent signal lamps was by no means uniform across the network, and despite electrification, signal operation was still quite manpower intensive. It was well after the initial introduction of electric signal lamps that automated wayside control systems came into widespread use.
Naturally, these control and monitoring systems were designed to work with the vast numbers of incandescent lamps that were in existence at the time. In addition to controlling the signal lights to display appropriate signals for current railroad conditions, another critical function of such systems was to proactively detect incandescent bulb failure. In addition to signaling the need for maintenance, detection of bulb failure would lead to changes in lamp control to safely accommodate a failed bulb.
Tests for proper operation executed by wayside control systems included: a cold filament test, a hot filament test, an inrush current test and a lamp off test. The cold filament test was performed when the incandescent lamp was off. A short current pulse (milliseconds) was applied by the control system to the lamp. An intact filament would mirror a short circuit to ground, and indicators of a passed test include pulse voltage below a specified voltage limit and pulse current above a specified current limit. The hot filament test applied when the lamp was on by monitoring operating current, with current falling below a specified limit indicating a failure. The inrush current test applied during the period immediately following energizing the incandescent lamp, when due to lower filament resistance, current levels can be much higher than the normal operating current. This inrush current effect is mitigated if the incandescent light is flashed since the filament does not cool immediately after turning off. The lamp off test applied while the incandescent bulb was on to verify that it could be turned off, with the control system looking for voltage across the bulb to go to zero. The off period was sufficiently short that the voltage was reapplied before the filament dimmed appreciably.
Light emitting diodes (LEDs) have very different operating characteristics than incandescent lamps. Perhaps most notably, an LED does not use a filament to generate light, and its current and voltage response characteristics when energized, operated and de-energized are very different from incandescent bulbs. Additionally, for adequate visibility an array of many LEDs must ordinarily be used to replace a single color incandescent bulb in a signal lamp. A failure of a limited number of LEDs within such an array may still fall within acceptable parameters.
Again due to the large number of existing wayside control systems in existence, it is considered desirable to continue using these extant wayside control systems when replacing incandescent bulbs in signal lamps with LED lamps. Accordingly, the LED lamp needs to assess its own functionality according to parameters suitable for LED lamps, while mimicking the corresponding response to tests designed for incandescent bulbs based on that assessment.
Complicating the matter further, multiple manufacturers produced wayside control systems with some significant variations in how one or more of the previously described tests were implemented. Some control systems make use of signal lamps having dual filaments, where a second filament is powered off a second power line after the first filament fails, allowing a period of continued operation after lamp failure is indicated to the control system by the failure of the first filament. Consequently, LED replacement lamps have needed to be specific to a particular wayside control system with which they were designed to work. Also, while most of these wayside control systems utilize programmable, processor-controlled units, there still exist older devices which utilize large mechanical relays. Additionally, most wayside control systems are provided with battery back-up, and some power the signal lamps via battery power for extensive periods of time, resulting in further complications for LED signal lamps.
Thus, while LED lamps exist for replacing incandescent lamps in some of these situations, further improvements are possible.